In aerial photography cameras, the length of the film is generally determined as a function of the thickness of the film and the radius or diameter of the roll of film. As auxialiary means for this purpose there are known dial gauges which are generally arranged in the cover of the film magazine and are connected to an indicator unit. The indication is subdivided either in meters of the supply of film or in numbers of exposures.
Various film thicknesses are taken into consideration by order of magnitude by various scales on the dial gauges.
Disadvantages are the low indication precision which results because the actual thickness of the film is insufficiently taken into account and because of the limited precision of the lever transmission. Read-out is not possible at a central operating unit, but only on the film magazine itself.
In magnetic tape technology (DE-OS No. 2918544) it is known to determine the remaining length of a tape from the determination of the rotation periods of a take-up spool and a feed spool. In order to determine the necessary starting data in these systems, however, there is required a measuring run with subsequent rewinding of the tape. Such a measuring run with subsequent rewinding is not possible in the film magazine of an aerial photography camera, which is provided with a drive operating in only one direction.